A conventional telephone hand set operates in the full duplex mode, which means that the parties can transmit and receive at the same time. This is possible because a conventional hand set is designed to have minimal acoustic coupling between the speaker and microphone. A handsfree system, which allows the user to operate the telephone without a hand set, has difficultly operating in the full duplex mode because of the high degree of acoustic coupling between the microphone and loud speaker. Most handsfree systems are therefore operated in the half duplex mode, which means that some form of switching must be provided in the transmit and receive paths.
In a conventional handsfree circuit, variable complementary loss attenuators are inserted in the transmit and receive paths. These are controlled in accordance with the audio signals in the two paths. Level detectors determine the overall noise level in the respective paths and speech detectors distinguish between background noise, which is assumed to be a generally constant level, and speech, which is generally characterized by short bursts of higher level audio. One example of such a system is described in U.S. Pat. No. 4,490,582.
Prior art designs have generally required filters in the transmit and/or receive audio paths to band limit the speech path to the 400 to 3500 Hz region. These filters are required to reduce background noise other heavy machinery, but they create a "hollow" effect that degrades the quality of the transmitted sound.
A further problem with the prior art circuits stems from the fact that the speech detectors work by detecting short bursts of sound. Consequently, many prior art circuits will respond to intermittent background noise, such as typing, causing the attenuators to ramp up to the full transmit state when no speech is present.
Yet another problem with the prior art design is the frequent presence of echo signals due to trans-hybrid reflection or reverberation in the room. Peak detector decay rate is made long so as to prevent reverberation echo from causing the handsfree to switch states.
An object of the present invention is to alleviate the aforementioned problems with the prior art.